Teamwork is the key to success for the seven wastewater treatment

plants serving the Twin Cities area of Minnesota and operated by Metropolitan Council Environmental Services (MCES). The violation-free performance of the Hastings and St. Croix Valley treatment facilities is an outstanding example.

The two plants haven't had a violation in more than 20 years. The Hastings plant, which discharges to the Mississippi River, serves about 20,000 residents of the City of Hastings and parts of surrounding Marshan Township. The St. Croix Valley plant serves 30,000 residents of Stillwater, Bayport and Oak Park Heights and discharges to the St. Croix River, a federally protected waterway. Both facilities provide secondary treatment, and both have received national and local awards.

Dennis Lindeke, assistant business unit manager whose responsibility for MCES includes the Hastings and St. Croix Valley plants, says both facilities benefit from shared services like training, operations, maintenance and quality control. Most recently, the regional approach has led to significant energy savings and reduced operational costs. "We are fortunate to be part of the larger organization," he says.

Observes Patricia Oates, business unit manager with MCES, the plants benefit from the internal expertise of the Metropolitan Council and its financial support, as well. "In the past, the Hastings plant staff would fill more than 1,500 sandbags to protect against flooding from the Mississippi River," she says. "In 2011, MCES funded the construction of a new flood wall, which would have been very difficult for a plant that size to pay for on its own."

Hastings plant

The Hastings plant, which treats about 1.4 mgd (design flow is 2.34 mgd), is actually operating on borrowed time, and doing it very well. "There were plans to close this plant and move treatment to a location outside of town," says Lindeke. "But the economic recession stunted area growth, and we've delayed the relocation until around 2020."

Built in 1955, the Hastings facility was expanded and renovated in 1983-86 at a cost of just over $8 million. Additional odor control was added in 1989 to help the plant fit in with its downtown location.

Wastewater enters through a bar screen with 1/2-inch openings and passes to grit removal. Primary sedimentation occurs in clarifiers covered with domes for odor control. The overflow is then directed to aeration basins equipped with Sanitaire fine-bubble ceramic diffusers (Xylem). The aeration tanks are also covered for odor control. Odorous air is treated in an activated carbon scrubber system (Calgon Carbon).

The final clarifiers are not covered, and the clarified effluent flows to chlorination (bleach) and dechlorination with sodium bisulfate. The treated water discharges to the Mississippi through a submerged outfall, easily meeting effluent standards of 25 mg/L BOD and 30 mg/L TSS.

Primary and waste activated solids are co-thickened by gravity and hauled as slurry to MCES' 250 mgd Metro Treatment Plant in St. Paul for dewatering and incineration. In 2011, a total of 5.8 million gallons was transported.

Property for the relocation of the plant has already been purchased, but now that the plant will remain at its current site for the next eight years or so, the MCES engineering department is examining what is required to continue cost-effective operation. "The most immediate need is new variable-frequency drives for the pumps and blowers," says Lindeke. "The ones we have are 20 years old and we can no longer get parts."

St. Croix Valley plant

The St. Croix Valley plant in the Village of Oak Park Heights is the only MCES facility discharging to the St. Croix River, a federally designated Wild and Scenic River. That's one reason the plant was the first MCES facility to install UV disinfection, in 1993.

The plant was built in 1959 and upgraded or expanded in 1970, 1973 and 1993. With a design capacity of 4.5 mgd, it handles an average of 3.2 mgd. Flow comes to the plant entirely by gravity, passing through a Vulcan climber bar screen on its way to a PISTA grit removal system (Smith & Loveless). Two open primary clarifiers provide sedimentation before fine-bubble aeration, secondary clarification and UV disinfection (Infilco Degrement). Alum is added for phosphorus removal.

Primary and secondary solids are handled the same as at Hastings. Last year, the plant transported 6.7 million gallons.

As at Hastings, odor control is a prime concern. "We're located near a boat marina and a very nice condominium development," says Lindeke. The pretreatment and sludge thickening facilities are covered, and odorous air is treated in carbon scrubbers — three large dual beds provided by Calgon. "We used to have complaints about our sludge truck area, as well," says Lindeke. "So we've covered the loading hatch on top of the trucks and take the air to the carbon scrubbers. It has worked like a charm."

Performance is consistently within the permit values of 24 mg/L for BOD and 20 mg/L for TSS. In fact, the plant won the U.S. EPA award for best mid-sized, advanced plant in the United States in 2001. The Hastings plant was nominated for the same award and was an EPA Region 5 award winner in 1989. Both facilities were instrumental in the MCES  winning the 2012 George Burke Jr. Facility Safety Award from the Central States Water Environment Association.

Teamwork for savings

These days, energy costs are a major concern in wastewater treatment operations across the country, and the regional approach is starting to pay significant dividends in energy savings at Hastings and St. Croix Valley. "As an organization, we set a 2006 baseline, with a 25 percent energy reduction goal for 2015, and a 50 percent reduction target for 2020," says Lindeke.

Between 2006 and 2012, the seven MCES treatment plants, including Hastings and St. Croix Valley, achieved energy cost reductions of about 15 percent, or $3 million a year. A big chunk of the savings came from improvements in the aeration system at the Metro Plant, and innovations like switching to fuel-efficient Toyota Prius vehicles throughout the MCES fleet.

However, energy savings measures at the smaller plants have contributed, as well. At Hastings, the adoption of fine-bubble aeration in 2001 produced about a 25 percent drop in energy usage. Installation of a Jones+Attwood screenings press (Ovivo), demonstrated at the 1992 WEFTEC Conference and Exhibition, reduced landfill costs from $25,000 to $15,000 a year and resulted in a 2.5 year payback.

At St. Croix Valley, a similar screenings press was installed in 1996. The staff also fine-tuned the odor-control system, monitoring the airflow and air speed and optimizing the number of air changes needed. "It was over-designed for the current odor load, so we've reduced the number of air changes," says Lindeke. "Where we use to have three 125 hp odor-control vessels online, now we have two."

Operators also moved the alum application point to the head of the primary clarifiers, saving about $20,000 a year in chemical costs, while also reducing the BOD loading to the aeration process.

Years ago, a standby emergency generator was added in anticipation of Y2K because two separate electrical substations fed the plant, with no installed generator, a situation that wasn't considered reliable at the time. The generator now proves its worth by providing peak load control under contract with Xcel Energy, the local electric utility. "This effectively reduced our power bills by 10 percent," says Lindeke.

Overall, the MCES plants have increased their combined ENERGY STAR rating from 53 to 62, an achievement that merited the Xcel Energy Gold Award for the highest electrical savings among large industrial and commercial clients in 2011. "It represented the equivalent of removing 1,200 homes from the grid," says Lindeke.

Staffing for success

Both plants are staffed with a lead operator, who reports to Lindeke, and a team of plant operators — three at Hastings and four at St. Croix Valley. The plants are staffed from 6 a.m. to 4 p.m. weekdays, and one operator works on site during weekends and holidays. The ties to the larger MCES organization are important in operations, maintenance and training at each facility.

The metro region maintains a new operator trainee program that assures a source of skilled operators to fill open positions. "We bring prospective operators into our program beginning with classroom training, followed by a probationary period where the new operator spends time out in the plants," says Lindeke. "It's a six-month training period."

The classroom work is designed around the Sacramento State University training program. The onsite experience involves shadowing veteran operators and doing everything from lab work and lubrication routes, to driving the sludge truck, to mowing the grass.

After their training, the trainees must complete a 90-day certification period at their eventual assigned plant. Prospective hires also need to become familiar with all of the operations manuals specific to a particular plant. At the end of the training period, the new operators bid, via seniority, for open positions within MCES. The organization also conducts aggressive recruiting, which helps fill staffing voids across the region.

The regional affiliation helps with maintenance, too. Maintenance is centralized out of the Metro Plant and is provided either by the central staff or by outside contractors. "We have electricians and mechanics on staff," Lindeke says. "But if we don't have the expertise on staff, or it's not available, then we'll contract it out as needed.

"It's a nice marriage. We use a computerized asset management system (WAM by Oracle). It's a complete program that transmits work requests to our Maintenance Business Unit (MBU) and also provides procurement and timekeeping functions. The MBU then plans, schedules and assigns the work. It works well."

Performance counts

Even with support from the region, it's the operators at the plant who make the everyday decisions that determine treatment performance. Lindeke, who started out at the Hastings plant as a summer intern in 1975, says the on-site staff uses Hach test kits and lab data to make key process decisions on the spot.

"At Hastings, we chlorinate the return sludge periodically to deal with filamentous growth," he says. "At St. Croix Valley, however, it seems that the alum inhibits the growth of filamentous bacteria." The key to success is the appropriate mixed liquor concentration and resulting food-to-mass ratio.

"We like to stay in the nitrification mode even though we're not required to nitrify by permit at either plant," Lindeke says. "The process is very stable when we nitrify, and at the lower food-to-mass ratio there is better oxygen transfer. Plus, the sludge settles nicely." Operators raise the mixed liquor solids levels in winter to stay in the nitrification mode in colder temperatures.

For microbiological monitoring, both plants use phase contrast microscopes. "They give a much better image of the bugs — protozoa — and mixed liquor," Lindeke says. "Our guys stay in tune with what the bugs should look like, how happy they are, and any early warning signs of changes that may need to be addressed. Your bugs are like the canary in the coal mine."

Lindeke fully credits the operators for the perfect records at both plants. He makes the rounds to the plants almost every day and maintains face-to-face contact with the plant staffs. "The lead operator at each plant is my primary contact," he says. "We see what's up and we talk about the process, maintenance and personnel needs.

"Our secret? Trained, motivated, accountable operators. We don't micromanage. We give them the tools to do the job and then get out of the way."